There are numerous sensor and analytical devices known in the art based on the use of optical signals. These devices are generally of the following three types of devices.
The first type of device employs indicator compositions which interact with the gas or other selected component being detected and measured and/or interact with a source of incident radiant energy. The indicator either fluoresces in response to the presence of the component and/or light signal, or causes some other change which is detected as an indication of the presence and concentration of the selected component. Examples of devices utilizing indicator compositions include those disclosed in U.S. Pat. Nos. Re. 31,879 to Lubbers, 3,612,866 to Stevens, 4,321,057 and 4,399,099 to Buckles, 4,476,870 to Peterson et al., and 4,737,343 to Hirschfeld.
There are a number of problems associated with the use of indicators. The indicator compounds and compositions are basically dyes which have specific characteristics, such as a particular fluorescence under particular conditions or particular interaction with other materials. As with any dye composition, it is extremely difficult to obtain a dye which does not bleach when exposed to the light used or does not change chemically over time. Changes particularly occur when the indicator composition is exposed to a working environment, which may severely and adversely affect the fluorescent or other properties of the indicator. When the indicator composition properties change, its fluorescent indications change, often in an unpredictable way. It is difficult to compensate for such changes in an indicator in order to maintain accurate analytical results, particularly because it is usually not known exactly when or to what extent an indicator undergoes a change. Thus, erroneous analytical results can be obtained from the sensor until the change in the indicator is detected and compensated for or the indicator replaced. Reliability of sensor devices utilizing indicator compositions is difficult to achieve, particularly long term.
Other disadvantages of these devices result from the fact that many of the indicator compositions are difficult to make and the fact that additional problems are encountered in placing the indicators in suitable polymers or adhering them to materials in such a way that they are immobilized and useful on a continuing basis in an analytical device. See 4,657,736 to Marsouer, et al., 4,712,865 to Hsu et al., and 4,849,172 to Yafuso, et al. Another disadvantage, which many of the above devices have, results from sensor systems that make use of the technique of quenching the fluorescence of the indicator. In such a system, the indicator gives its strongest signal or fluorescence in the absence of the component being detected and measured. When that component such as oxygen, is present, the fluorescence of the indicator is quenched or decreased, thus providing an inverse response of the fluorescent light to the presence of the compound being measured. In many applications, a direct response is desired in order to achieve a desired sensitivity.
The second type of device is based on absorbance of light or radiant energy. Examples of absorbance type devices are described in U.S. Pat. Nos. 4,096,388 to Wong, 4,201,222 to Haase, and 4,800,886 to Nester. Generally, the compound in question is received into a chamber or a polymer body where an incident light source is directed into the chamber or polymer body where a compound is present. The absorbance of light in the polymer body or in the chamber varies with the amount of the compound present. The remaining, unabsorbed light is detected and the concentration of the compound is then correlated to the amount of absorbed light. In this type of system, the difficulty is in achieving the desired combination of sensitivity and selectivity. The incident energy can be absorbed or lost due to unknown reasons or is absorbed by other components that are present other than the one of interest, thus giving rise to false positives and inaccurate indications.
The third type of device is based on the fluorescence of the component itself under an incident light source. See U.S. Pat. Nos. 3,795,812 to Okabe and 4,272,486 to Harman. In these devices, the component is received into a chamber in gas phase, irradiated with a light source to cause the component to fluoresce, and the fluorescent light detected to measure the presence of the component. A difficulty encountered with this type of device is that other undesired materials present in the chamber can also fluoresce in response to the incident light, thus giving false positives and erroneous indications. Harmon discloses a system for pretreating the gas samples to convert or remove such undesired materials before analyzing the sample in the chamber.
In view of the above, disadvantages and deficiencies in the prior art devices, and for other reasons, it is an object of this invention to provide a sensor apparatus and method which eliminates the necessity of using indicator compositions and thereby eliminates the associated problems presented by indicator compositions.
It is another object of this invention to provide a sensor apparatus and method having improved sensitivity and having improved selectivity to eliminate false indications.
It is another object of this invention to provide a sensor apparatus and method which is self-compensating and thereby capable of giving accurate indication of component concentrations, automatically compensated for variations in the input energy signal or energy level or other variations in the system.
It is another object of this invention to provide an analytical system which enables the practical use of preferred semiconductor laser light sources in fiber-optic sensor devices.